M. Stuke

Sub-picosecond UV laser ablation of metals

Laser ablation of Nickel, Copper, Molybdenum, Indium, Tungsten and Gold by short ultraviolet laser pulses (0.5 ps, 248 nm) in vacuum is reported for the first time. For Nickel and Indium, ablation is also studied in air to demonstrate the influence of the ambient atmosphere. Metal ablation in air is significantly less efficient than in vacuum due to redeposition of ablated material. The ablation rates in vacuum are discussed using a thermal model, which also allows to estimate ablation rates for other metals from basic optical and thermal properties. A comparison of the morphology of ablation sites after nanosecond and sub-picosecond ablation shows unequivocally the advantages of short-pulse laser ablation for high-precision patterning of thermally good conducting materials in micron-scale dimensions.

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient αeff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.

Time resolved dynamics of subpicosecond laser ablation

The ablation of lithium niobate (LiNbO3), poly(tetrafluoroethylene) (PTFE, teflon), poly(methylmethacrylate) (PMMA) and polyimide (PI) by 500 fs UV excimer laser pulses at 248 nm is reported. Time‐resolved measurements were carried out with pulse pairs of variable delay in the range from −200 to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses, and —depending on the material and fluence—can increase or decrease for very short time delays. For LiNbO3, efficient shielding is observed within a few picoseconds. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half‐maximum below 400 fs, since two‐photon absorption dominates the ablation process. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps.

Subpicosecond ultraviolet laser ablation of diamond: Nonlinear properties at 248 nm and time‐resolved characterization of ablation dynamics

Ablation of diamond by short UV pulses is reported. The ablation threshold for 500 fs pulses at 248 nm was found to be 0.6 J/cm2 (equal to a peak intensity of 1.1 TW/cm2). Intensity‐dependent transmission measurements from low intensities to 100 GW/cm2 showed that two‐photon absorption is the dominant nonlinearity in this range and the two‐photon absorption coefficient was measured to be (1.6±0.3) cm/GW. In the intensity regime above threshold (≳1 TW/cm2), however, low ablation rates indicate strong absorption by laser induced free carriers. The dynamics of diamond ablation was investigated time‐resolved using pump probe transmission and pulse pair ablation measurements. Ablation with short pulses at 248 nm is also compared to ablation with 15 ns pulses at 193 nm.

Laser-Driven Movement of Three-Dimensional Microstructures Generated by Laser Rapid Prototyping

Three-dimensional microstructures consisting of aluminum oxide and aluminum were fabricated by laser-induced direct-write deposition from the gas phase. Trimethylamine alane and oxygen were used as precursors. Thermal expansion forces resulting from suitable laser irradiation were used to drive the movement of microstructure parts. Applications include micromechanical actuators, such as microtweezers and micromotors. The one-step nature of the laser direct-write process allows rapid prototyping of such devices.

Towards a general procedure for sequencing single DNA molecules.

In this paper we report on the latest technical advances towards single molecule sequencing, a useful method currently developed especially for fast and easy de novo sequencing. Different approaches for complete labeling of DNA with fluorescent dyes are described. In addition, the experimental set-up for the sequencing process is shown. We demonstrate the ability to purify the buffer and enzyme solutions. Inorganic buffers were purified down to at least 20 fM of remaining fluorescent impurities. The exonuclease buffer solution could be cleaned down to 0.8 pM whereby its full activity was kept. Finally, we show a selection procedure for beads and present the data of a model experiment, in which immobilized DNA is degraded by an exonuclease within a polymethylmethacrylate (PMMA) microstructure. Furthermore, the mathematical processing of the obtained raw data is described. A first complete experimental cycle is shown, combining all preparatory steps which are necessary for single molecule sequencing in microstructures.

Highly efficient single molecule detection in microstructures.

For DNA single molecule sequencing, the complete detection of all dye-labeled monomers which are cleaved off during the sequencing reaction is an essential requirement. In this work we address the feasibility of single molecule detection in microstructures with a confocal multi element set-up. We present statistical data on single molecule recognition and explain a refined data evaluation technique for single molecule burst analysis. From these data the signal-to-noise ratio in microstructures is evaluated as well as the overall detection efficiency. So far, detection efficiencies of single molecule events of up to 60% have been shown in microstructures.

Tunable UV laser photolysis of organometallics with product detection by laser mass spectroscopy: Trimethylaluminum.

We use tunable UV laser light in the region 200–320 nm, produced by frequency doubling the output of a dye laser, for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum Al(CH3)3. Only the TMA monomer absorbs UV light for λ >220 nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum atoms Al plus organic fragments, and aluminummonomethyl AlCH3 molecules plus organic fragments. The ratio [Al]/[AlCH3] is wavelength dependent. We present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al, Ga, In).

Argon-ion laser direct-write Al deposition from trialkylamine alane precursors

Laser direct writing of aluminum lines on Si, GaAs and Al2O3 substrates using the trialkylamine alane precursors, (C2H5)3N · AlH3 (TEAA) and (CH3)3N · AlH3 (TMAA), is reported. Deposition, carried out with a continuous wave Ar+ laser working at 514 nm, results from the thermal decomposition of the precursor via laser-induced surface heating at temperatures in excess of 360 K. It proceeds in two steps: surface nucleation, which occurs typically in 0.01 to 0.1 s depending on the laser power and the substrate nature; and the successive Al growth. Uniform lines are produced at scanning speeds up to 300 μm/s. Higher deposited heights are obtained with TMAA due to its higher vapor pressure when compared to TEAA. 7 μm wide, 0.7 μm thick Al lines are generated by a single surface scan at a scanning speed of 100 μm/s and a high power density (1.55 MW/cm2). Passing N2 or He gas through a TEAA bubbler increases the deposited height by a factor 1.6 in comparison to deposition without the bubbler. Al lines deposited from TEAA and TMAA are polycrystalline. The rate of crystallite growth is greater than the rate of nucleation, leading to an increase of the surface roughness with the line thickness. Adhesion of the deposit to the substrates is good and Al lines with resistivities down to 4.5 μΩ cm, i.e. twice the bulk value are obtained.

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (“incubation”) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (“ablation”) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 μm. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 μm in diameter up to now, given by the mask.